Oh actually, now I think about it I once took a 45-minute train journey for the princely sum of £1 because it was after the station ticket office had closed and I didn't see a single official between walking into station A at one end and walking out of station B at the other. You were meant to guess how much the ticket is worth and put that amount in the platform-ticket machine before getting on the train, but I had no idea and in any case didn't have much change. This was before they had automatic ticket machines which can sell you an actual ticket for your journey (and it was a fairly small station anyway).

I wrote to one of the train companies concerned and they graciously said they wouldn't bother to charge me the difference.